The present invention generally relates to retort pouches for materials, such as food products. More particularly, the present invention relates to a retort pouch which is suitable for wet food and which can be easily opened and reclosed.
Closable pouches having an integral zipper or fastener are well known in the art of packaging dry foods. These pouches are often rectangular, made of a web material, and have heat-sealed portions at both side and bottom edge portions. A top opening portion is often provided with means for repetitively opening and reclosing the opening, such as a zipper fastener fastened to the inner surfaces thereof, and extending from one side edge to the opposite side edge. The top opening portion is usually heat-sealed until the product is to be used. In order to open the pouch, the pouch is manually cut between the top heat-sealed portion and the zipper portion, and the top edge is removed. If any product remains in the pouch after being opened, the pouch can be reclosed and reopened by means of a zipper comprising a ridge and trough arranged in registration on the inner surface of the front and back panels of the pouch.
One pouch manufacturing technique is described in U.S. Pat. No. 5,542,902 to Richison et al. In this process, a single web is divided into two portions, with one portion being placed beneath the other. A zipper closure and a base gusset are oriented between the two portions, and then front and back panels of the pouch (formed of the two slit portions of the web) are heat-sealed to one another along their edges to form the pouch.
However, when pouches are to be used to hold retort food, an oxygen blocking capability and adequate strength at the high sterilization temperatures are required. It has been disclosed that these goals am attainable by forming the wall portions of the pouch from a laminated film consisting of four layers, the outermost of which is polyester (hereinafter xe2x80x9cPETxe2x80x9d), followed by aluminum foil (hereinafter xe2x80x9cAlxe2x80x9d), then BiOriented Nylon (hereinafter referred to as xe2x80x9cBONylxe2x80x9d) and the innermost of a non-oriented or cast polypropylene film (hereinafter xe2x80x9cCPPxe2x80x9d). It is also known to provide a laminated film consisting of the same four layers of material in the following arrangement: outermost is PET, then BONyl, then Al, and the innermost layer of CPP. The foil layer provides a barrier to air, humidity and light and therefore should preferably be preserved intact until the product reaches the consumer.
The feature of stand-up capability has been achieved by adding a separate gusset between the bottom parts of the two webs, or by adding a flap to the bottom of a single sheet which is intended to be folded in two, thus forming two leaves with the flap as a gusset serving as a base of a stand-up pouch. The aforementioned laminated films have sufficient strength so that pouches made from them withstand the high temperatures and pressures generated during the retort process without bursting and releasing their contents or otherwise losing their hermetic sealing and loss of the required food sterility. This means, however, that great force is required in order to tear them, so as to open the pouch in the first place. Thus, generally a knife or scissors are required in order to cut the laminated film.
To address this problem of difficulty of opening, it has been suggested to utilize a tape formed of an oriented plastics material at a position between the top edge and the zipper. The oriented tape is arranged for easy cutting in one direction. However, placing such tapes on the pouch reduces the efficiency of the pouch producing process, and results in higher costs per pouch.
It is also known, for example from U.S. Pat. No. 3,790,744 to Bowen, to utilize laser energy to provide a weakened line in the laminate web material, so as to form a tear line in the laminate, and from EP 0 473 517 to American National Can Company to provide laser scoring in two close parallel lines across a non-reclosable laminate pouch to create a tear line. Generally, the laser scoring is performed on the individual webs while they are in the slitter, before assembly of the pouch. When it is time to assemble the pouch, there is often a problem of ensuring that the score lines on the front and back panels of the pouch are in registration with one another, which is required in order to obtain good tear performance.
However, as described by Richison, it is extremely difficult to ensure the precision of registration required for a good tear. The teachings of EP 0 473 517 attempt to compensate for this lack of precision by providing two or three close parallel laser scored lines, about xe2x85x9xe2x80x3 to {fraction (1/16)}xe2x80x3 apart on the same surface. While this might address the tear performance caused by imprecisely aligned webs, it requires the use of two or three times as many lasers, thereby greatly increasing the expense of the production process. In addition, the disclosed method of scoring of two lines so close to one another can detrimentally affect the mechanical properties of the pouch, thereby affecting its suitability for use with wet materials in a retort process, due to the extreme conditions generated by the process.
An alternative method of facilitating the removal of the top portion of the pouch is disclosed in EP 0 345 930 to Kabushiki Kalsha Hosakawa Yoko where it is taught to subject the inner layer of the laminate to surface roughening treatment by an abrasive roller for forming a surface roughened zone of small width. The mechanical strength of the laminate in this surface roughened zone is reduced, so as to facilitate cutting of the sheet therealong. In theory, this treatment is effected only on the internal cast polypropylene layer or on the outer polyester layer. In practice, this is problematic since it applies an abrasive mechanical means to the laminate. Since it is very difficult to control the depth of the recesses cut by the abrasive, this can lead to the creation of pinholes in the adjacent air impermeable layer, thereby permitting air and moisture to contact the contents of the pouch.
Known stand-up pouches, which are intended for retort, are mostly suitable only for use with dry contents (i.e. having moisture content significantly below 2%, usually as little as 1.2% or less). Examples of dry foods are dry dog food, dried soup mixes, dried gravy mixes, dried, cake mixes, dried pudding mixes. This is so because the use of flexible retort packages (instead of conventional cans) for liquid or partially liquid foods (for example having moisture content greater than 2%), such as soup, gravy, purse, sauces, toppings, condiments, etc., poses particular challenges. Usually, food material is packed in a retort pouch and then transferred to an autoclave where it undergoes a sterilization process at high temperatures, above 121xc2x0 C. and pressure of about 2 atm. The volume of water increases many hundreds of times when it turns into steam and the energy released by the steam is much higher than that released by the water at 100xc2x0 C. Thus, in packages with liquid or wet products, the pressure and steam temperatures produced inside the package even by a small quantity of evaporated water are much higher than with dry foods, dramatically increasing the resistance requirements of the components used, including the different webs, adhesives and dyes. For this reason, at present, most of the canning industry utilizes metal cans, whose resistance at high temperatures and pressures are greater than those of flexible, plastic webs. During the retort process with flexible packages, it is necessary to work with special autoclaves wherein it is possible to apply opposing, external, pressure of about 2 atm, as well as tight control over the cooling phase, in order to prevent the packages from bursting.
The particular sensitivity of the stand-up pouch to the pressures created during the retort process, as compared with other packages, arises from the complicated make-up of the pouch, and the various heat-seals required to assemble the base of the pouch. The pouch may include a plurality of overlapping webs, requiring heat-sealing through multiple layers. These points are particularly vulnerable to bursting under pressure during the retort process.
Thus, the high requirements for these plastic webs have prevented the incorporation of fastener means for repetitively opening and reclosing of such packages which, in themselves, reduce the resistance of pouches to the retort process. For example, the use of a zipper-type fastener is known to weaken the heat-seals on the side edges of the pouch. This problem was addressed recently at an international seminar titled xe2x80x9cStand up Pouchesxe2x80x9d, presented by La industria de Allmentos, held in Mexico D.F. on Nov. 4, 1998, in a lecture by Ing. Jorge Pasquel Santacruz, in which it was explicitly stated that, xe2x80x9cit is important to know that when one decides to use a zipper like any other feature, it is not possible for the pouch to be sterilized by the method of retort.xe2x80x9d
Accordingly, there is a long-felt need for easy to open, reclosable, stand-up pouches useful for wet retort, and it would be very desirable to have a method for forming pouches which are easy to open, yet can be repeatedly reclosed and reopened.
According to the present invention, there is provided a pouch formed of laminated sheets heat-sealed along their side, top and bottom edges, with a fastener sealed between the laminated sheets, and a heat-sealed upper portion above the fastener, characterized in that the pouch can be sealed with wet contents and withstands retort temperatures and pressures generated without bursting.
In another embodiment, the pouch constructed in accordance with the present invention, is characterized in that at least one laser-scored tear line is provided on each sheet in registration with a corresponding laser scored tear line on the opposing sheet, the score lines being located between the top edge and the fastener and extending substantially between the side edges.
Alternatively, a pouch is formed by folding over in half and sealing a single sheet of laminate The sheet may have a small flap at one end thereof, the flap permitting formation of a gusset to serve as a base of a stand-up pouch.
As an additional alternative, a long roll of laminate has adhesively positioned thereon and adjacent to one long edge thereof a strip of fastener of equal length. The laminate is then folded on itself along its longitudinal centerline to form two leafs and a gusset is formed by the inward folding up and in of the fold line in between the leafs. The bottom, gusseted end is then heat-sealed and the sides and top are then repetitively and sequentially heat-sealed and cooled in accordance with the inventive process described in further detail hereinbelow. The laser score lines can be applied in accordance with this third alternate embodiment either before the first folding of the laminate or after the final welding of the sides and top edge portion. According to one embodiment, the distance between the laser-scored tear line and the fastener is between about 5-30 mm.
For purposes of clarity it helps to consider the retort pouch as divided by the fastener into two sections: i.e. a storage section, containing the desired contents of the pouch, nominally below the fastener, between the fastener and the bottom side, and an opening section, used for facilitating the reclosing and the reopening of the pouch, nominally above the fastener, between the fastener and the top side.
While most pouches are rectangular in shape, pouches made in accordance with the present invention could be triangular, circular, oval or of other shapes. Edge terms used herein are merely to facilitate the presentation of this invention without intending to limit it in any way to shapes having a clearly defined top, bottom and two sides.
There is also provided in accordance with the present invention an exemplary method for forming a stand-up pouch including forming a pouch from two laminated sheets, heat-sealing a bottom portion with a gusset between the two sheets, heat-sealing the two sheets along their side edges together with a fastener positioned and sealed between the two side edges of the laminated sheets, and laser scoring the two laminated sheets to form a single tear line on each sheet, the tear lines being in registration with one another.
Alternatively, an exemplary method for forming a pouch comprises the steps of folding a single sheet over in half, thereby forming two leaves, inserting a zipper, heat-sealing and laser scoring the leaf surface above the zipper to form at least one tear line on each leaf, said tear lines being in registration with one another.
According to yet another exemplary embodiment of the invention, the two sheets are scored with a laser on the inside surface of the pouch before the step of heat-sealing a bottom portion.
According to still another exemplary embodiment of the invention, the two sheets are simultaneously scored in registration by a laser on the outside surface of the pouch after the step of heat-sealing the two sheets along their bottom edges, or after heat-sealing of the sides.
According to yet a further embodiment of the invention, the shoots are scored both on their inner and on their outer surfaces.
According to yet another exemplary embodiment of the invention, both inner and outer surfaces are scored simultaneously by a single laser.
According to a preferred exemplary embodiment of the invention, the periphery of the pouch is heat-sealed in three stages, each at a successively slightly, higher temperature, and then cooled in two stages.
There is also provided in accordance with the present invention a machine for producing pouches, the machine including a web supply unit, a pouch forming unit, a fastener insertion unit, a longitudinal sealing unit, a servo drive unit for advancing the web from unit to unit, and a transverse sealing unit including at least one heat-sealing bar and at least one cooling bar, characterized in that the machine further includes at least one laser scoring system including preferably a CO2 laser mounted adjacent the cooling bars, and mirrors and other optics arranged as is known and necessary to focus the CO2 laser on respective first and second webs forming a pouch in registration with one another, so as to form a laser-scored tear line.
Laser, mirrors and optical elements can also be provided to form laser score lines on both inner and outer web surfaces simultaneously.
According to an exemplary embodiment of the invention, the machine includes three heat-sealing bars, each at a successively slightly higher temperature and pressure than the previous bar, and two cooling bars.
Further according to a preferred embodiment, the servo drive includes a roller having an annular groove arranged in registration with fasteners on partially formed pouches.